That's because you are actually dealing with two separate mind-numbingly painful problems with any jellyfish sting, Yanagihara says. First, the umpteen-thousand microscopic tubules imbedded in your skin have already spewed a ton of venom. And second, they are ready to unleash even more into you.

Peeing on yourself—or a friend, lover, or really unfortunate stranger in need—only helps with the latter issue. Urine has a naturally low pH, which inhibits the cells from discharging more venom (and can also remove any whole tentacles that are hooked into your skin). While vinegar (another popular treatment option) also has a low pH, urine does have two serious advantages: It's warm and readily available.

And if you're too shy to whip it out on the beach, just never, under any circumstance, try rubbing, pressure, or compresses (seriously, you're just pushing the venom deeper into your skin!) or using an EpiPen, Yanagihara warns. As soon as you're stung, your body reacts by pumping huge amounts of epinephrine and norepinephrine through your bloodstream. Add any more and you could OD—or just not feel any extra relief.